Chromium-enriched diffused aluminide

ABSTRACT

A method of applying a protective coating to an article comprises the steps of a) depositing aluminum in a surface region of an article, and b) depositing chromium is the surface region of the article subsequent to step a), whereby at least a portion of the chromium replaces at least a portion of the aluminum. Another method and an article are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims priority to U.S. Provisional PatentApplication No. 62/026,143, filed Jul. 18, 2014.

BACKGROUND

This disclosure relates to chromium-enriched metallic coatings.

Coatings are used to enhance corrosion resistance of gas turbine enginecomponents, or other components in severe environments. Coatings can be,for example, aluminum coatings or chromium coatings.

SUMMARY

In a featured embodiment, a method of applying a protective coating toan article comprises the steps of a) depositing aluminum in a surfaceregion of an article, and b) depositing chromium is the surface regionof the article subsequent to step a), whereby at least a portion of thechromium replaces at least a portion of the aluminum.

In another embodiment according to the previous embodiment, prior tostep b) there is a first amount of aluminum in the surface region of thearticle, and subsequent to step b) there is a second amount of aluminumin the surface region of the article, the second amount less than thefirst amount.

In another embodiment according to any of the previous embodiments, atleast one of the aluminum and the chromium are deposited by chemicalvapor deposition.

In another embodiment according to any of the previous embodiments, thechemical vapor deposition is performed at a temperature of betweenapproximately 1900° F. and 2100° F. (1037.78° C. and 1148.89° C.) for atime of between approximately 1 and 6 hours.

In another embodiment according to any of the previous embodiments, stepb) is performed with an activator.

In another embodiment according to any of the previous embodiments, theactivator is a halide activator.

In another embodiment according to any of the previous embodiments, stepa) results in an aluminum layer between approximately 0.5 and 3.0 mils(0.01 to 0.08 mm) thick.

In another embodiment according to any of the previous embodiments, stepb) results in a chromium layer having approximately 20-40% chromium byweight in an outer 30 to 60% of the coating thickness.

In another embodiment according to any of the previous embodiments, stepb) results in a chromium layer substantially free of alpha chromium.

In another featured embodiment, a method of applying a protectivecoating to an article comprises the steps of a) vapor depositingaluminum in a surface region of an article formed of a low-chromiumnickel-based alloy, b) vapor depositing chromium in the surface regionof the article while removing at least a portion of the aluminum usingan activator, and c) heating the article such that the chromium replacesat least a portion of the aluminum through action of the activator.

In another embodiment according to the previous embodiment, step c) isperformed at a temperature of between approximately 1900° F. and 2100°F. (1037.78° C. and 1148.89° C.) for a time of between approximately 4and 20 hours.

In another embodiment according to any of the previous embodiments, theactivator is a halide activator.

In another embodiment according to any of the previous embodiments, theactivator interacts with the aluminum such that chromium replaces thealuminum.

In another featured embodiment, an article comprises a base alloy, acorrosion-resistant coating on the base alloy, the corrosion-resistantcoating containing a region of approximately 20%-40% chromium, and analuminum diffusion zone disposed between the base alloy and thecorrosion-resistant coating.

In another embodiment according to any of the previous embodiments, theregion extends between a depth of approximately 5 to 60 microns (0.2 to2.3 mils) from a surface of the coating.

In another embodiment according to any of the previous embodiments, theregion is a first region, and further comprising a second regionextending from the first region to the aluminum diffusion zone, thesecond region having a thickness of approximately 0.6 mils (15 microns)and comprising less than approximately 10% chromium by weight.

In another embodiment according to any of the previous embodiments, thealuminum diffusion zone comprises less than approximately 20% chromiumby weight.

In another embodiment according to any of the previous embodiments, thearticle is a turbine blade.

In another embodiment according to any of the previous embodiments, thearticle comprises external and internal surfaces and wherein the coatingis applied to at least one of the external and internal surfaces.

In another embodiment according to any of the previous embodiments, thebase alloy is at least one of a nickel-based superalloy and alow-chromium superalloy.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of the present disclosure willbecome apparent to those skilled in the art from the following detaileddescription. The drawings that accompany the detailed description can bebriefly described as follows.

FIG. 1 illustrates an example article.

FIG. 2 illustrates an example method of applying a protective coating toan article.

FIG. 3 illustrates a representative micrograph of a chromium-enrichedcoating.

FIG. 4 illustrates another representative micrograph of achromium-enriched coating.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an example article 100. As will bedescribed, a method 200, shown in FIG. 2, can be employed to apply aprotective coating to the article 100.

In FIG. 1, the example article 100 is an airfoil having one or moreexternal surface regions 102 and one or more internal surface 104regions. The external surface region 102, the internal surface region104, or both are coated with a protective coating 106. In anotherexample, only a portion of the external and internal surface regions102, 104 are coated with the protective coating 106. The airfoil can bean airfoil for a gas turbine engine, such as a blade or vane. However,it is also to be understood that the features descried herein areapplicable to other types of articles as well.

The article 100 may be a metallic article formed of a base alloy. Forexample, the base alloy is a superalloy. In a further example, thesuperalloy is a nickel-base alloy. In a still further example, the basealloy is a low-chromium superalloy, such as a superalloy having lessthan approximately 12% by weight chromium.

The steps or actions described with respect to the method 200 to formthe coating 106, shown in FIG. 2, can be employed with additional stepsor other processes as desired. In Step 202, aluminum is deposited in oneor more surface regions 102, 104 of the article 100 (aluminizing) Thealuminizing is accomplished by chemical vapor deposition (CVD) ordiffusion coating, for instance.

In a further example, the article 100 is exposed to an aluminum sourcematerial such as a chromium aluminum alloy in the presence of anactivator, such as a halide activator, and a cover gas such as argon orhydrogen, at a temperature of between approximately 1900° F. and 2100°F. (1037.78° C. and 1148.89° C.) for a time of between approximately 1and 6 hours. The halide activator can be, for instance, a fluoride or achloride in a solid or gaseous form.

In another example process, the aluminum or aluminum source andactivator can be combined with another material such as aluminum oxidepowder prior to being deposited on the surfaces 102, 104 of the article100. The other material can enhance the deposition of aluminum or theproperties of the deposited aluminum. Then, the article 100 is exposedto the aluminum or aluminum mixture at a temperature of less thanapproximately 1500° F. (815.56° C.) for a time of between approximately2 and 4 hours.

In another example process, the aluminum or aluminum source andactivator can be combined with another material such as aluminum oxidepowder prior to being deposited on the surfaces 102, 104 of the article100. The other material can enhance the deposition of aluminum or theproperties of the deposited aluminum. Then, the article 100 is exposedto the aluminum or aluminum mixture at a temperature of between 1500° F.and 1900° F. (815.56° C. and 1037.78° C.) for a time of betweenapproximately 2 and 8 hours. In another example process, the aluminum oraluminum source and activator can be combined with another material suchas aluminum oxide powder and a binder to form a slurry prior to beingdeposited on the surfaces 102, 104 of the article 100. The othermaterial can enhance the deposition of aluminum or the properties of thedeposited aluminum. Then, the article 100 is exposed to the aluminum oraluminum mixture at a temperature of between 1500° F. and 1900° F.(815.56° C. and 1037.78° C.) for a time of between approximately 2 and 8hours.

Any of the processes described above can be repeated to incorporateadditional aluminum multiple times. Additionally, any of the processesdescribed above can form an aluminum coating on the surface 102, 104 ofthe article 100 that is between approximately 0.5 and 3.0 mils (0.01 to0.08 mm) thick, for example.

In Steps 204-208, chromium is deposited onto article 100 (chromizing).Chromizing is accomplished by CVD or diffusion coating, similar to theprocesses discussed above with respect to aluminizing, in one example.The chromium is applied to the article 100 from a pure chromium sourcematerial in the form of powder nuggets with an activator such as ahalide activator in Step 204. The activator may be, for example,ammonium chloride, chromium chloride, or another chloride or fluoride.Prior to step 204, the chromium can be combined with aluminum oxide,which can enhance the deposition of chromium or the properties of thedeposited chromium. In Step 206, the article 100 is heated totemperature of between approximately 1900° F. and 2100° F. (1037.78° C.and 1148.89° C.) for a time of between approximately 4 and 20 hours. Theheat up to temperature can coincide with the deposition, or be conductedseparately. During the heating process, the activator interacts with thealuminum to allow the chromium to replace at least some of the aluminumin the coating to produce a chromium-enriched coating 106. That is,subsequent to the chromizing process of Steps 204-208, there is lessaluminum in the surface regions 102, 104 of the article 100. In oneembodiment, the chromium-enriched coating 106 is substantially free ofalpha-chromium.

FIGS. 3-4 show representative micrographs of a chromium-enriched coating106. Areas 1-17 represent example areas of the coating 106 which wereanalyzed. The coating 106 includes a first region 108 at a surface 107of the coating 106. The first region 108 extends from approximately 5microns (0.2 mils) below the surface 107 of the coating 106 to a depthof approximately 2.3 mils (60 microns) and comprises approximately20-40% chromium by weight. In one example, the first region 108comprises approximately an outer 30 to 60% of the coating thickness.

The coating 106 further includes a second region 110 under the firstregion 108. The second region 110 is approximately 0.6 mils (15 microns)thick and comprises less than approximately 10% chromium by weight.

The coating 106 further includes an aluminum diffusion zone 112 underthe second region 110 which comprises less than about 20% chromium byweight. The aluminum diffusion zone 112 is disposed between the secondregion 110 and the base alloy of the article 100. The aluminum diffusionzone 112 is a region of aluminum deposited during the aluminizingprocess of Step 202 but not replaced by chromium during the chromizingprocess of Steps 204-208. That is, the aluminum concentration in theportion of the aluminum diffusion zone 112 adjacent the second region110 can have a higher aluminum concentration than the first and secondregions 110, 112. The aluminum concentration in the aluminum diffusionzone 112 decreases with distance to the base alloy of the article 100.

Although a combination of features is shown in the illustrated examples,not all of them need to be combined to realize the benefits of variousembodiments of this disclosure. In other words, a system designedaccording to an embodiment of this disclosure will not necessarilyinclude all of the features shown in any one of the Figures or all ofthe portions schematically shown in the Figures. Moreover, selectedfeatures of one example embodiment may be combined with selectedfeatures of other example embodiments.

The preceding description is exemplary rather than limiting in nature.Variations and modifications to the disclosed examples may becomeapparent to those skilled in the art that do not necessarily depart fromthe essence of this disclosure. The scope of legal protection given tothis disclosure can only be determined by studying the following claims.

What is claimed is:
 1. A method of applying a protective coating to anarticle, comprising the steps of: a) depositing aluminum in a surfaceregion of an article; and b) depositing chromium in the surface regionof the article with an activator subsequent to step a), whereby at leasta portion of the chromium replaces at least a portion of the aluminum,wherein step b) results in a chromium layer having approximately 20-40%chromium by weight in an outer 30 to 60% of the coating thickness. 2.The method of claim 1, wherein there is less aluminum in the surfaceregion of the article subsequent to step b) than prior to step b). 3.The method of claim 1, wherein at least one of the aluminum and thechromium are deposited by chemical vapor deposition.
 4. The method ofclaim 3, wherein the chemical vapor deposition is performed at atemperature of between approximately 1900° F. and 2100° F. (1037.78° C.and 1148.89° C.) for a time of between approximately 1 and 6 hours. 5.The method of claim 1, wherein the activator is a halide activator. 6.The method of claim 1, wherein step a) results in an aluminum layerbetween approximately 0.5 and 3.0 mils (0.01 to 0.08 mm) thick.
 7. Themethod of claim 1, wherein step b) results in a chromium layersubstantially free of alpha chromium.
 8. A method of applying aprotective coating to an article, comprising the steps of: a) vapordepositing aluminum in a surface region of an article formed of alow-chromium nickel-based alloy; b) vapor depositing chromium in thesurface region of the article while removing at least a portion of thealuminum using an activator; and c) heating the article such that thechromium replaces at least a portion of the aluminum through action ofthe activator, wherein steps b) and c) result in a chromium layer havingapproximately 20-40% chromium by weight in an outer 30 to 60% of thecoating thickness.
 9. The method of claim 8, wherein step c) isperformed at a temperature of between approximately 1900° F. and 2100°F. (1037.78° C. and 1148.89° C.) for a time of between approximately 4and 20 hours.
 10. The method of claim 8, wherein the activator is ahalide activator.
 11. The method of claim 8, wherein the activatorinteracts with the aluminum such that chromium replaces the aluminum.12. A method of applying a protective coating to an article, comprisingthe steps of: a) depositing aluminum in a surface region of an article;and b) depositing chromium in the surface region of the article with anactivator subsequent to step a), whereby at least a portion of thechromium replaces at least a portion of the aluminum, wherein step b)results in a chromium layer substantially free of alpha chromium. 13.The method of claim 12, wherein at least one of depositing the aluminumand depositing the chromium includes forming a slurry prior to thedepositing.
 14. The method of claim 12, wherein there is less aluminumin the surface region of the article subsequent to step b) than prior tostep b).
 15. The method of claim 12, wherein at least one of thealuminum and the chromium are deposited by chemical vapor deposition.16. The method of claim 12, wherein the activator is a halide activator.17. The method of claim 12, wherein depositing the chromium includesforming a slurry prior to the depositing.